Web Forms :: Select All Dropdownlist?

im working on Visual Studio 2010, and im making a .net web page. The final idea is to use filters by using dropdownlist. By now i have 3 dropdownlist in cascade. The first filters the second and so on. Then by a grid view i display the selected items, right now i need to add an item that allows me to select all the items in the dropdownlist so in the next filters i like i have selected all. My code filters everything but i cant add this item. I only work in asp .net, not in the code behind wich i think is #C if im not wrong... i have to add a button in each dropdown.

I have read others post and the say that i have to add this in the dropdown>> "appenddatabounditems="True" and this sql sentence

WHERE ([PROJID] = CASE WHEN @PROJID= -1 THEN [PROJID] ELSE @PROJID END)

but it doesnt work.

heres the code.

<form id="form1" runat="server">
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:TestingConnectionString %>"
SelectCommand="SELECT [PROJID] FROM [sxb_Proyectos] ORDER BY [PROJID]">
</asp:SqlDataSource>
<br />
<asp:Label ID="Label1" runat="server" Text="Proyecto"></asp:Label>
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1" AppendDataBoundItems="true"
DataTextField="PROJID" DataValueField="PROJID">
<asp:ListItem Value="0"><Select an option></asp:ListItem>
<asp:ListItem Value="-1"><Select All></asp:ListItem>
</asp:DropDownList>
<br />
<br />
<br />
<br />
<asp:SqlDataSource ID="SqlDataSource2" runat="server" ConnectionString="<%$ ConnectionStrings:TestingConnectionString %>"
SelectCommand="SELECT DISTINCT Ax50_Empleados.emplid, Ax50_HorasRegistradas.EMPLID
FROM Ax50_Empleados
INNER JOIN Ax50_HorasRegistradas
ON Ax50_Empleados.emplid = Ax50_HorasRegistradas.EMPLID
where PROJID = @PROJID
ORDER BY Ax50_Empleados.emplid">
<SelectParameters>
<asp:ControlParameter ControlID="DropDownList1" Name="PROJID" PropertyName="SelectedValue"/>
</SelectParameters>
</asp:SqlDataSource>
<asp:Label ID="Label2" runat="server" Text="Rut"></asp:Label>
<asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource2"
DataTextField="emplid" DataValueField="emplid">
</asp:DropDownList>
<br />
<br />
<br />
<br />
<asp:SqlDataSource ID="SqlDataSource3" runat="server" ConnectionString="<%$ ConnectionStrings:TestingConnectionString %>"
SelectCommand="SELECT DISTINCT sxb_Producto.Producto, Ax50_HorasRegistradas.LINEPROPERTYID
FROM sxb_Producto
INNER JOIN Ax50_HorasRegistradas
ON sxb_Producto.Producto = Ax50_HorasRegistradas.LINEPROPERTYID
where emplid = @emplid and PROJID = @PROJID
ORDER BY sxb_Producto.Producto">
<SelectParameters>
<asp:ControlParameter ControlID="DropDownList2" Name="emplid" PropertyName="SelectedValue"/>
<asp:ControlParameter ControlID="DropDownList1" Name="PROJID" PropertyName="SelectedValue"/>
</SelectParameters>
</asp:SqlDataSource>
<asp:Label ID="Label3" runat="server" Text="Producto"></asp:Label>
<asp:DropDownList ID="DropDownList3" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource3"
DataTextField="Producto" DataValueField="Producto">
</asp:DropDownList>
<br />
<br />
<br />
<br />
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
DataSourceID="SqlDataSource4"
EmptyDataText="There are no data records to display.">
<Columns>
<asp:BoundField DataField="PROJID" HeaderText="PROJID"
SortExpression="PROJID" />
<asp:BoundField DataField="EMPLID" HeaderText="EMPLID"
SortExpression="EMPLID" />
<asp:BoundField DataField="LINEPROPERTYID" HeaderText="LINEPROPERTYID"
SortExpression="LINEPROPERTYID" />
<asp:BoundField DataField="QTY" HeaderText="QTY" SortExpression="QTY" />
<asp:BoundField DataField="TXT" HeaderText="TXT" SortExpression="TXT" />
<asp:BoundField DataField="TRANSDATE" HeaderText="TRANSDATE"
SortExpression="TRANSDATE" />
</Columns>
</asp:GridView>
<asp:SqlDataSource ID="SqlDataSource4" runat="server"
ConnectionString="<%$ ConnectionStrings:TestingConnectionString %>"
ProviderName="<%$ ConnectionStrings:TestingConnectionString.ProviderName %>"
SelectCommand="SELECT [PROJID], [EMPLID], [LINEPROPERTYID], [QTY], [TXT], [TRANSDATE], [TRANSSTATUS], [DIMENSION], [DIMENSION3_] FROM [Ax50_HorasRegistradas] where PROJID = @PROJID and emplid = @emplid">
<SelectParameters>
<asp:ControlParameter ControlID="DropDownList3" Name="Producto" PropertyName="SelectedValue"/>
<asp:ControlParameter ControlID="DropDownList1" Name="PROJID" PropertyName="SelectedValue"/>
<asp:ControlParameter ControlID="DropDownList2" Name="emplid" PropertyName="SelectedValue"/>
</SelectParameters>
</asp:SqlDataSource>
</form>